For the past 20 years, time domain reflectometry has been used to measure the volumetric moisture content of soils (volume of moisture per unit volume of soil), mostly in the field of soil science. As shown in FIG. 1, these measurements involved the insertion of a probe 10 comprising a central rod 12 and two or more peripheral rods 14 into the soil 16 to be measured. The peripheral rods 14 (which are preferably three in number) are spaced equidistant from the central rod 12 and equidistant from each other. A coaxial transmission line 18 is then coupled to the structure with the center conductor of the coaxial cable 18 coupled to the center rod 12 and the exterior shield (outer conductor) of the coaxial cable 18 coupled to each of the peripheral rods 14. In this way, the peripheral rods 14 simulate the effects of a continuous outer coaxial shield in the soil 16, without the requirement of attempting to drive a cylindrical probe into the soil 16. Time domain reflectometry analysis equipment 20 is then coupled to the coaxial cable 18, and the reflections of high frequency electrical signals from the soil 16 are measured using the analysis equipment 20. These reflections will change in predictable ways depending upon the dielectric constant of the soil 16, which has been found to be strongly correlated with the volumetric moisture content of the soil 16. Therefore, time domain reflectometry has been established as a viable tool for measuring volumetric moisture content of a soil.
The prior art probes such as those illustrated in FIG. 1 are intended for permanent installation at a measurement location with periodic measurements being made through the probe 10 over a period of time. Physically, the prior art probes 10 are not rugged enough to withstand repeated insertion into and extraction from hard soils. The prior art probes 10 are not suitable as portable probes to be used for rapid insertion and removal following one-time soil measurement at a variety of locations within a soil field to be measured. There is therefore a need for a probe design which is rugged enough to withstand repeated insertions and extractions from dense soil, thereby facilitating the taking of one-time measurements at multiple locations. The present invention is directed toward meeting this need.
Although time domain reflectometry techniques are useful in measuring volumetric moisture content of soils, they cannot be presently used to measure gravimetric moisture content of soils (weight of moisture per unit weight of soil solids). Many applications in geotechnical engineering require a knowledge of the gravimetric moisture content of soil. In order to convert from the volumetric moisture content measured by time domain reflectometry to the gravimetric moisture content, it is necessary to know the density of the soil. There are several prior art methods for measuring in-place density in moisture content of soils, such as the sand-cone method, the rubber balloon method and the drive tube method. These methods are difficult and time consuming and are usually accompanied by the oven drying method of measuring moisture content in order to measure in-place dry density and moisture content of the soil. The oven drying method of measuring moisture content requires a significant waiting time before measurement results are available. Another method, the nuclear method of measuring in-place soil moisture content and density requires extensive calibration. Moreover, the nuclear method is potentially hazardous because it utilizes radioactive materials. There is therefore a need for a technique to measure in-place gravimetric moisture content and density quickly, precisely, and preferably in a non-destructive manner. The present invention is directed toward meeting this need.